1. Field of the Invention
The present invention relates to an automatic transmission and, more particularly, to a hydraulic servo unit for applying/releasing frictional engagement elements of the automatic transmission.
2. Related Art
In automatic transmissions have cylinders of hydraulic servo units arranged in a back-to-back relationship, it is customary (as in Japanese Patent Laid-Open No. 8851/1983) that the return springs for returning the pistons of the hydraulic servo units are arranged within the spaces interior to the inner circumferences of the individual frictional engagement elements, with each spring having one end connected to the cylinder and its opposite end connected to the piston.
In such an arrangement, the frictional engagement elements and the return springs are arranged with their respective inner and outer circumferences axially overlapped. Such an arrangement is effective to minimize the axial length for the combination of the frictional engagement elements and their hydraulic servo units. However, this arrangement cannot always be said to be rational taking into consideration the entirety of the automatic transmission, including the arrangement of the shift elements, such as the planetary gears to be connected by those frictional engagement elements.
In the pursuit of a rational arrangement for compactness and improved mountability of the automatic transmission, the shift elements, such as planetary gears, could be arranged radially inward of one frictional engagement element. However, the space interior to the hydraulic servo unit associated with the shift elements is occupied by the shift elements so that no room remains in that space for the return springs, contrary to the previously described arrangement.